Loud-speaker



June 1, 1943- v s. w. BLESSING LOUDSPEAKER Filed Sept. 30, 1940 Snnen 01: Gregory VlfBlessL'r zg (Itforneg mine-.11.... 1, 1943 LOUD-SPEAKER Gregory W. Blessing, Moorestown, N. 1., asslgnor to Radio Corporation of America, a corporation of Delaware Application September 30, 1940, Serial No. 359,130

Claims.

This invention relates to improvements in loudspeakers.

The field structure of the usual loudspeaker comprises a yoke, a yoke plate, a magnet, and -a pole piece which projects through an orifice in the yoke .plate with suflicient clearance to provide 'an air gap within which the voice coil is adapted to move. In accordance with the usual practice, the several stationary parts are formed into a composite structure by a spot" or other welding process. Thus, the yoke plate is commonly welded to the free ends or arms of the yoke, the magnet tothe inner surface or U of theyoke and the pole pieceto the free end of the magnet. 1

Irrespective of the advantages claimed for suchv welded magnetic structures, it may be said generally that welding isobjectionable since it subjects the metal to crystallization adjacent the areas at which the magnet is welded to the yoke. Metal thus crystallized has decreased permeability and the over-all eflect of these imperiections may be reflected in a decrease in power output, since obviously the total magnetic flux adjacent the air gap within which the voice coil moves is affected thereby.

Accordingly, it is an object of'the invention to provide a loudspeaker enjoying a highfactor of merit and one having a field structure free from crystallized areas which may adversely aflect the magnetic flux adjacent its air gap.

Another object of the invention is to. provide a strong, durable and inexpensive loudspeaker and one which, by reason of its simplicity and economy of parts, lends itself readily to mass production methods. a

The foregoing and other objectsare achieved inaccordance with the invention by the pro-' through the longitudinal axis of a loudspeaker constructed in accordance with the principle of the invention,

Figure 2 is a view in perspective of the yoke in the loudspeaker of Fig. 1 and showing an unbonded end of sheet metal of which the said part is constructed,

Figure 3 is a fragmentary view of one form" of apparatus upon which the prefabricated sheet metal may be wound to form a yoke similar to the completed yoke shown in Fig. 1, and

Figure 4 is a side elevational view showin a tool for shaping the prefabricated sheet metal structure of Fig. -3 in its ultimate, preferred form.

In the drawing, wherein like reference characters designate the same or corresponding parts in all figures, F designates generally the field structure and H the cone housing of a loudspeaker. The field structure is a' composite structure comprising a yoke i, a yoke plate In.

a magnet 3 mounted within the yoke and a pole piece 5 which is mounted on the magnet and projects into an orifice lin the top portion -or "plate lp of the yoke.

As shown more clearly in the partly exploded view of Fig. 2, the yoke I and the yoke plate lp comprise a convolution formed from a single strip of metaLpreferabIy. hot rolled steel (say .031 thick), having two or more turns (in this case, five) intimately bonded one to another by means of an interposed coating, layer or film of a synthetic resinous material 9.

The thermoplastic fusion method described in my copending application Serial No. 359,129. filed Sept. 30, 1940, has been successfully employed in bonding the turns of the yoke and in uniting the several parts I, 3 and i into a composite structure. Thus, referring now to Figs. 3 and 4, in constructing the yoke, a clean, fiat metal strip is which had previously been punched with holes la, lb, etc., at suitable intervals, was coated with hydrolyzed polyvinyl acetate (a 28 percent by weight solution of polyvinyl acetate in .methyl acetate) which was then thoroughly dried and then wound upon a circular mandrel II which was provided with a pin it for engaging and centering the holes (in, lb,

. etc.) in the strip. The strip thus formed was removed from the mandrel and shaped on a rectangular expanding block is (which had its corners waxed in order to prevent the formation of a permanent bondbetween the block and the inner surface of the strip during the subsequent bonding operation). The clamping pressure necessary to the formation of the bond was applied to this structure by expanding the expanding block (as by inserting a pin, not shown, in the tapered opening ll) against the restraining force of a clamp, not shown. Thenecessary heat was supplied by a muiile type furnace; As described and claimed in the above identified copending application, during the bonding operation the synthetic resinous material was subjected to a temperature greatly in excess of that required to render it fluid and sufllcient to effect partial pyrolysis of said material, yet not so great as to cause complete pyrolysis of said material.

A clamp (not shown) having a cylindrical projection for establishing the required spacing or air gap between the pole piece and the inner surface of the orifice I, was employed in bonding the pole piece 5 to the magnet 3 and the magnet to the yoke I by the said thermoplastic fusion method." This clamp was provided with'jaws which engaged the inner and outer surfaces of the yoke and with a thumbscrew capable of exerting the requisite bonding force upon the exposed end of the pole piece and magnet.

In a test of the mechanical strength of the field structure F of the loudspeaker of Fig. 1, several hammer blows of substantial intensity were required .to shatter the thermoplastic bond between the magnet 3 and the yoke l, whereas a ingle blow of similar intensity served to shatter the welded bond between the magnet and the yoke in a loudspeaker of standard construction. The remarkably stronger construction of the loudspeaker of Fig. 1 may be attributed to the application to this art of a bond covering the entire surface area between the parts, whereas, in accordance, with the prior art construction the area actually bonded comprised but a few spaced pointsor spots" adjacent which the magnetic material (Alnico) had become crystallized (with a resultant loss both in strength and in magnetism) apparently from normal exposure to the welding process.

Theoretically, it might appear that the layers of bonding material between the several turns of the yoke would result in a decrease in the amount of the magnetic flux in the air gap as compared to that which obtains in the air gap of a solid yoke plate of the same thickness. practice, however, the opposite appears to be the case. In accounting for this unusual fact, it may be that any theoretical disadvantage is offset by the fact that no part of the magnetic material is subject to crystallization as in the prior art method of assembly. Further, the total thickness dimension of the bonding layers is extremely small (say .001" per turn) so that the total non- I magnetic area in the air gap may be no greater than that which obtains between the unwelded areas between the parts of prior art structures.

ing sense except as required by the prior art and by the spirit of-the appended claims.

' What is claimed is:

1. In a loudspeaker, a composite field structure comprising a unitary yoke and yoke plate, a magnet and pole piece within the yoke, said parts being intimately bonded each to another with a partially pyrolyzed thermoplastic material.

said strip having a plurality of openings ther'in spaced from each other at distances such that,

when said turns are completed, said openings are in axially aligned relation for reception of a pole 5 piece, and a layer of partially pyrolyzed hydrolyzed polyvinyl acetate between said turns constituting a thermoplastic bonding agent'for bond ing said turns to each other into an. integr unit. J

3. The method of forming a unitary yoke and yoke plate for a loudspeaker which comprises forming a plurality of openings in a strip of magnetic material in longitudinally spaced relation along said strip, applying to said strip a layer of thermoplasticbonding material, turning said strip around 3n itself longitudinally to bring said openings into axially aligned relation with each other to thereby provide a convolution having a plurality of turns, and subjecting said v convoluted strip to a temperaturesufiicient to cause said bonding material to bond said turns to each other.

4. The method of forming a unitary yoke and yoke plate for a loudspeaker which comprises forming a plurality of openings in a strip of magnetic material in longitudinally spacedrelation along said strip, applying to said strip a layer of hydrolyzed thermoplastic bonding material,

turn-ing said strip around onitself longitudinally to bring said openings into axially aligned-relation with each other to thereby provide a'convolution having a plurality of turns, and subjecting said convoluted strip to a temperature in excess of that required to render said bonding material fluid, said temperature being suiilcient to eifect partial pyrolysis of said material but not sufliciently great to cause complete pyrolysis of said bonding material, whereby to bond said turns to each other. I

5. The method of forming a unitary field structure for a loudspeaker which comprises forming a plurality of openings in a strip of magnetic material in longitudinally spaced relation along said strip, applying to said strip a layer of thermoplastic bonding material, turning said strip around on itself to bring said openings into axially aligned relation with each other to provide a plurality of turns, subjecting said convoluted strip to a temperature in excess of that required to render said bonding material fluid and effective to only partially pyrolyze said bonding material whereby to bond said turns to each other, applying to that portion of the inner turn of said bonded strip which is opposite to said a'xially aligned openingsa'nd i axial alignment with said openings amagnet with the aid of an interposed layer of said bonding materialainserting into said openings in jconcen'tric relation therewith a pole piece of m netic material having a length greater than -the distance between said openings and the adjacent face of said ma'gnet and having a smaller diameter than's'aid openings whereby to provide an'annular' airgap between said pole piece and the portions of said piece to said adjacent magnet face with an interposed layer of said bonding material, andjfinally subjecting said strip portion, said magnet: and said pole piece to substantially the same temperature whereby to bond said magnet and'said pole piece to each other and to said convoluted strip. s

' GREGORY W. BLESSING.

strip defining said openings, applying said "pole 

